Helicopter altitude control

Collective, also termed "collective pitch", is the part of a helicopter's flight control system that converts the rotation of its rotor blades into a controllable amount of lift. The helicopter pilot operates this system with the collective stick, which in its most basic form is a control stick that can be pushed forwards or backwards to raise or lower collective.

One of the common misconceptions about controlling a helicopter's altitude (i.e. whether it rises, falls or hovers) is that it is accomplished by simply increasing or decreasing the engineRPMs and thus the speed the rotor blades spin at. This is not the case: lift is controlled by the overall pitch of the rotor blades. Although modern helicopters frequently have a system whereby throttle is automatically increased or decreased with collective application (to keep the rotor speed as constant as possible), this is dependent upon the particular helo and beyond the scope of this writeup.

Unlike cyclic control, which alters the individual pitch of each rotor blade as it rotates, collective control alters the pitch of all the rotor blades at once, increasing or decreasing them all collectively. This is accomplished by a system of swash plates.

A swash plate is a thick ring of metal that surrounds the rotor shaft - there are two of these: the lower swash plate (LSP) and the upper swash plate (USP). The LSP is stationary and mounted on top of actuating rods that can push it up or down, and the USP is fixed to the rotor blades, rotating with them. The USP bears more or less directly on the LSP so when the LSP tilts, the USP tilts with it. Because the USP rotates while the LSP does not, the pitch angle of the USP will oscillate up and down as it rotates over the LSP, if the LSP's angle is anything other than horizontal.

The USP is fitted to the rotor blades such that when it tilts, it causes the rotor blade above the tilting portion to alter pitch. The tilting motion is applied to the rotor blades by a series of rods, each of which link the USP to an assembly that each rotor has at its root called a "pitch horn". The pitch horn is an arm that protrudes from the leading edge of the rotor blade root. It can be moved up or down to rotate the rotor blade along its length, pronouncing or flattening its pitch.

The collective is relatively simple in its operation compared to the cyclic: operation of the collective throttle simply raises or lowers the entire LSP by a uniform amount all the way around. Increasing the collective will raise the entire LSP by an equal amount: this exerts a more or less equal force on the USP which in turn operates the pitch horn on each rotor blade by the rod attached to it. This increases the pitch of each rotor blade by an equal amount. Reduction of collective will lower the LSP and effect an overall reduction in rotor pitch.

Bear in mind that collective changes occur on top of any cyclic control being applied and vice versa. For example, if no cyclic control is applied (in which case the swash plates would be level), the effect of increasing the collective would be as follows:

Just the lower swash plate is shown (the dotted horizontal line) here. One can see how the vertical rods support the plate from below with ball joints ('A'). Now, if the collective is increased the following will result:

The entire LSP has risen (exaggerated for illustrative purposes - in actual fact the effects of all the rotor control systems are very subtle). This in turn will raise the USP, exerting an upward force on the rods attached to the pitch horns on the rotor blades, turning the rotors along their length so that their leading edges are higher than their trailing edges. Now, let's say some cyclic control is being applied, causing the swash plate to pitch in one direction (crudeified for ASCII art limitations):

...this will have the effect of increasing the overall, or average pitch of the rotor blades regardless of any cyclic control. Lift in general will increase but cyclic control will still take effect. Further, a given amount of cyclic control will have a stronger effect with increased collective, because this increases the angle of attack ('wing' effect) of each rotor blade.

It is worth noting that past a point, increasing collective further will put the rotor blades at too high an angle of attack, creating more drag than lift. It's in this kind of situation that over-torquing of the rotor blades can result; in this situation it is increasingly difficult for the engine to maintain the speed of the rotor blades so the rotor speed decreases and the blades effectively stall, resulting in a loss of altitude.